Rotary blowers of the backflow-type, particularly Roots-type blowers, are characterized by noisy operation. Roots-type blower noise may be roughly classified into two groups: solid borne noise caused by clashing and clanging of components subjected to fluctuating loads, and fluid borne noise caused by fluid flow characteristics such as rapid changes in fluid velocity. The invention of this application is concerned with solid borne noise caused by timing gear rattle and with secondary noise caused by a spring damper.
The timing gears of the Roots-type blowers, such as the blower disclosed in U.S. Pat. No. 4,638,570, may produce an objectionable gear rattle when the meshed teeth of the gears are substantially unloaded. Such unloaded conditions occur during non-supercharging, low engine speed modes of operation. The gear rattle may be mitigated by a damper having a low torque transmitting capacity, low rate spring which is made flexibly operative only during such modes of operation by forward and reverse stops. However, objectionable secondary noise may be cause by impacts of the forward and reverse stops due to abrupt movement of the engine throttle and/or rough low speed engine operation, and by impacts of the reverse stop during engine shut-down.